In order to allow the speed of an induction motor to be changed, a polyphase AC supply of variable voltage and variable frequency is required, since induction motors operate at a speed substantially corresponding to the supply frequency and their flux is determined by the supply voltage. As illustrated in FIG. 1 and known from, e.g., Power Electronics and Variable Frequency Drives, edited by Bimal K. Bose, IEEE PRESS, 1996, pp. 211, such an AC supply is generally created from a DC source using an electronic DC-to-AC inverter. In the example shown in FIG. 1 the DC source is represented by a DC link capacitor 10 which is in turn fed by a three-phase rectifier 12 from a three-phase AC source not shown. The reference numeral 16 in FIG. 1 denotes the induction machine represented by its stator windings. The DC link capacitor 10 is a relatively large electrolytic capacitor which is inserted to keep the DC-link voltage u.sub.d constant and to provide a path for rapidly changing currents drawn by the inverter 14. It is normally preferred to use a diode bridge as rectifier 12 because of its low cost. However, a diode bridge is not capable of regenerating power to the AC supply. If an AC drive is able to operate in four quadrants, power will be regenerated to the DC link when the induction machine works in generating mode. The reversal of power flow will consequently raise the DC link voltage above its normal operation value, and steps must be taken to absorb this regenerated power to prevent a dangerous increase of the DC link voltage. Typically, a switched resistor (not shown) in parallel with the DC link capacitor 10 is used to absorb this energy. This makes extra power electronics and control electronics necessary, and thus increases the cost of the AC drive, and reduces its reliability.
To avoid using a switched resistor or any other energy dissipating hardware and, thereby, to reduce the cost, it is known to dissipate the regenerated energy in the AC motor and to some extent in the inverter. To facilitate understanding of the principle of such methods, it will be discussed at first which kinds of losses exist in an induction motor and how these losses can be used to dissipate the regenerated energy.
In order to assess the possibilities of dissipating energy in the induction machine, the various losses in the machine are first identified. These losses are illustrated in the power flow diagram of FIG. 2. For a 3-phase machine the input power is given by EQU P.sub.elec =3V.sub.1 I.sub.1 cos.phi. (1)
where V.sub.1 is the phase voltage,
I.sub.1 is the phase current, and PA1 cos.phi. is the displacement factor. PA1 1. Copper losses P.sub.cu,s in the stator windings which are proportional to the square of the stator current I.sub.s : EQU P.sub.cu,s =3I.sub.s.sup.2 R.sub.s (2) PA1 where R.sub.s is the ohmic resistance of one phase winding; and PA1 2. Stator core losses P.sub.Fe,s, which include hysteresis and eddy current losses and depend on the magnitude of the stator flux and the stator frequency f.sub.s. PA1 3. The copper loss in the rotor windings which is proportional to the square of the rotor current I.sub.r : EQU P.sub.cu,r =3I.sub.r.sup.2 R.sub.r (3) PA1 where R.sub.r is the ohmic resistance of one phase winding of the rotor; and PA1 4. The rotor core losses P.sub.Fe,r which include hysteresis and eddy current losses and depend on the magnitude of the rotor flux and the rotor frequency f.sub.r. At normal operating condition, the frequency f.sub.r of the rotor current is low and then these losses can be neglected.
Losses occur both in the stator and in the rotor. The losses in the stator are:
The power that remains after deducting P.sub.cu,s and P.sub.Fe,s from P.sub.elec is the air gap power P.sub.ag which crosses the air gap. A portion of it is dissipated as copper loss in the rotor windings and as core loss and in the rotor core, respectively. These losses are:
The remaining power is converted into mechanical power. A portion of it is lost as windage and friction losses, which losses depend on the rotor speed. The rest is finally the mechanical output power P.sub.shaft, which is the useful output power from the machine. It is assumed here for simplicity that the mechanical inertia of the rotor is contained in the mechanically coupled load. The rotor has then zero inertia and the accelerating or decelerating torque are contributed by the shaft power.
To brake an induction motor to reduce its speed, a braking torque is required to counteract the active torque generated by the mechanically coupled load and by the inertia of the drive train. The shaft power P.sub.shaft is negative during braking. The power flow is shown in FIG. 3. Depending on the sign of slip s, there are two modes of operation. The generating mode is shown in FIG. 3(a). The slip is negative (s&lt;0), which means that both the stator flux and the rotor rotate in the same direction, and the flux rotates slower than the rotor. As a result, the air gap power P.sub.ag is negative. Unless the motor operates at very low speed, the air gap power is larger than the stator losses such that the input power P.sub.elec is negative. In this case, all motor losses, and the input power P.sub.elec as well, are supplied by the shaft power P.sub.shaft.
In the plugging mode, the slip is positive (s&gt;1). The stator flux and the rotor rotate in different directions. FIG. 3(b) illustrates this situation. The stator losses are exclusively supplied by the inverter and the rotor losses are supplied both together by the inverter and by the shaft power.
In case of s=1 the stator frequency is zero. This condition characterizes the well known DC braking method.
A negative shaft power P.sub.shaft is called braking power in the following. At constant mechanical speed .omega., the developed braking torque is proportional to the shaft power P.sub.shaft. To make the braking torque as high as possible, maximum braking power must be achieved. If there is no energy consumed in the DC link, the braking power can be mostly absorbed in the motor, and to some extent in the inverter. Hence an efficient braking scheme should aim at maximizing the power dissipation in the motor, which maximizes the braking torque.
Considering the example of a 10-kW induction motor, the losses typically subdivide as follows:
TABLE 1 stator copper losses P.sub.CUs = 0.6 P.sub.total stator core losses P.sub.Fes = 0.25 P.sub.total rotor copper losses P.sub.CUr = 0.15 P.sub.total
P.sub.total are the total losses of the motor. The rotor core losses can be neglected since the rotor frequency equals the slip frequency and hence is very low. Friction and windage losses are also very small if forced ventilation is assumed.
It can be seen from the example that most of the motor losses occur in the stator. The rotor losses are small in comparison with the stator copper and stator core losses. It should be noted that this observation is generally true for any motor of any power rating.
The aforementioned DC braking is the method currently preferred in the art for braking an induction motor. In this method, the stator flux is first controlled to near zero and then a DC voltage is generated by the inverter to establish maximum DC current flow in the stator windings. Although the stator current has a maximum value in this case, the resulting braking power is very small. The stator flux does not rotate and hence the air gap power is zero. The stator copper losses are maximum, but they do not contribute to the braking power as they are supplied by the inverter. The stator core losses are zero since the stator frequency is zero. The rotor core losses are negligible since the flux is very small. Only the energy dissipated in the rotor resistances is supplied by the shaft power, and hence the braking power is low. Referring to the typical machine losses in Table 1, the rotor copper losses and hence the braking power amount to only 15% of the total machine losses. This means that the capability of the motor to dissipate the braking power is badly exploited.
Apart from the poor ability to develop braking torque, the method of DC braking exhibits also a poor dynamic performance, since the flux can not be abruptly changed. Before the braking torque is developed, a time delay of several times the rotor time constant .tau..sub.r elapses during which the flux decays to the required small amplitude. It takes the same time delay until the original flux is reestablished when returning to normal operation.
High dynamic performance is achieved using the principle of field oriented control (FO control) also known as vector control. Here, the torque producing or quadrature component i.sub.q (simply referred to "q-current" hereinafter) and the magnetizing or in-phase component i.sub.d (simply referred to "d-current" hereinafter) of the stator current are dynamically decoupled. FIG. 4 shows a block diagram illustrating the basic structure of an FO control. 20 in FIG. 4 denotes a speed controller, 22 a flux controller, 24, 26 and 50 adders, 28 a current controller, 32 and 34 coordinate transform elements, 36 a pulsewidth modulator, 38 a speed sensor and 40 a motor model or observer. 10 is an uncontrolled bridge rectifier, 12 is the DC link capacitor, and 14 the inverter. The induction motor is denoted by 16. 80 is a voltage transducer which generates a signal proportional to the DC link voltage u.sub.d. The voltage transducer 80 is not part of the known FO control scheme but is used in the present invention as will be explained in detail later.
Since the basic structure and operation of an FO control is well known (see for instance FIG. 3 in A. M. Khambadkone, J. Holtz: "Vector-Controlled Induction Motor Drive with a Self-Commissioning Scheme," IEEE Trans. on Industrial Electronics, Vol. 38, No. 5, October 1991, pp. 322-327), a detailed description is omitted here.
The motor enters the generating mode when the speed reference .omega.* is lower than the measured motor speed .omega.. The field, and consequently the stator core losses, are maintained at nominal values. Since the power regenerated to the DC link is zero, the airgap power equals the stator losses. The q-current and the rotor current are then very small. Their contributions to the copper losses can be neglected. It is only the d-current that produces copper losses in the stator. The d-current is typically 30% of the nominal stator current. Since the copper losses are proportional to the square of the current, the losses at field oriented braking are EQU P.sub.FO =P.sub.Fe s +0.3.sup.2 P.sub.Cu s =(0.25+0.05)P.sub.total =0.3P.sub.total (4)
This means that the motor dissipates only 30% of its rated losses at field oriented braking.
To maximize the stator and rotor copper losses, the stator and rotor currents must be maximum. The maximum stator core losses occur when the stator flux is maximum and the stator frequency is high. Moreover, all losses must be supplied by the shaft power, not by the inverter. Finally, high dynamic torque control must be ensured.